A web-offset printing press line may include a plurality of various components or modules operating on a substrate or web, typically paper. These components may include printing units, a chill stand including chill rolls, numbering units, gluers, slitters, and/or folders, as well as others. Independently driven motors drive rollers in the different modules of the press to drive the web along the press line.
Tension of the web is controlled by the use of gain. In other words, the rollers driving the web are operated at a slightly higher surface speed than the web. Generally, successive rollers downstream are operated at successively higher gain, so that tension is maintained between the driven points. The rollers are operated at gains which also compensate for the various treatments performed on the web: for instance, dryers will heat and expand the web, and chill rolls will cool and shrink the web. Gain is generally created through the use of harmonic gearboxes, as disclosed in U.S. Pat. No. 3,724,733, or by drag gearboxes, as disclosed in U.S. Pat. No. 5,269,222. The required gain is generally determined by the press speed, tension, operator input, or a combination thereof, and is typically on the order of two to five parts per thousand.
Excessive gain will cause the web to stretch, then slip. Because holding friction is greater than slipping friction, this tends to cause an oscillation. A so-called grip-and-slip action occurs where the web is pulled tighter and tighter until slippage occurs. Such oscillation is undesirable during normal press running operation, because the cutoff of the resultant printed product will similarly oscillate. During speed changes, however, product is usually not savable for other reasons, so larger gains during speed changes are often not a disadvantage. As a general rule, high tension, up to two times normal tension, will cause only paper wastage. However, low tension is also problematic, causing web-breaks and press stoppage. Press stoppage causes considerable delay and paper wastage upon re-start.
In the manufacture of large web-offset printing presses, one trend is to replace the lineshaft, which drives the various components of the press, with independent drive motors for each component. The various components, such as chill rolls, numbering units, gluers, slitters, and folders, can then be independently added to the press in a modular fashion. The mechanical gearboxes are replaced by software such that the motors driving the components are commanded to rotate at slightly higher speeds than the web, thereby achieving the required gain.
The programming of the independent motor drives on the press has emulated the operation of their predecessor, the lineshaft. Motors in such independently driven or "shaftless" presses are programmed to operate in synchrony with the printing units, with an operator-adjustable degree of gain. This gain is typically two to five parts per thousand.
The modular nature of shaftless presses also tends to require more idler rollers in the web path. The various components, often from different manufacturers, will typically accept a web at a standardized height and also output the web at a standardized height. A standardized height for the various components means that the web path is not optimized, requiring more idler rollers in the web path.
The idler rollers contribute to the inertia that must be overcome by the web during speed changes. For a large enough speed change and a large number of idler rollers, a web span may exhibit normal tension at one end of the span, yet be slack at the other end. A slack web generally causes excessive web breaks. For example, a slack web at a slitter will merely wrap around the slitter blade rather than be cut, causing tearing and web breaks. As another example, a slack web in a folder may move off-center laterally, causing jams during folding. Further, a slack web during a web acceleration at a chill roll may cause the web in the dryer to be sucked toward the dryer exhaust ports. The lateral shift of the web may be so extreme as to be beyond the ability of a web guide to correct, causing jam-ups and press stoppage. Web tension along a span may be substantially different than what is measured at a transducer at a single point in the span.
The drag of idler rollers on an accelerating web can also cause the position of the web to retard behind the position normally expected. This causes a cutoff error in downstream cutting equipment which results in improperly cut material that is unacceptable, increasing wastage.
A further complication of the idler rollers relates to the inertia reflected back to the drive motors. If the web is slack, no motor motion is transmitted to the idler rollers. If the web is tight, the idler rollers in the path of the web are seen by the drive motors as an inertial load. A drive motor that is programmed to react to a large inertial load will overreact and oscillate or chatter when the load is removed. Because this oscillation is undesirable, the drive motors are adjusted to react to the minimum inertial load. Under normal press running conditions, this inertial setting is inadequate, causing sluggish reaction time and inadequate tension control.
One method to reduce the effects of idler roller inertia is to utilize lightweight rollers such as Valcom (TM) carbon composite rollers available from American Roller Company of Bannockburn, Ill. These rollers are typically one-third the weight of similar aluminum rollers, but are twice the cost.
With rising postal rates and paper costs, publishers are utilizing lighter weight paper stock as a substrate for magazines and catalogs. This paper is more prone to wrinkling, particularly on press startup. Wrinkles may directly initiate a tear of the web with subsequent press stoppage. Wrinkles may also interact with the slitter to initiate a tear near the slitter.
Additionally, when a new roll of paper is spliced into the running web, the new paper is pulled more forcefully by the upper folder rollers of the press, resulting in a typical ten to thirty percent increase in tension. This tension increase decays logarithmically to nominal tension with a half-life on the order of about thirty seconds. Uncorrected, this tension increase causes an undesirable advancement of the cutoff of the resultant printed product. The cutoff disruption is particularly large on the upper web of a double-web press. A system to compensate for this tension disturbance is disclosed in U.S. Pat. No. 4,452,140. However, the disclosed system requires additional compensator stations, which may be difficult to retrofit onto a press.